Small peptides, e.g., some hormones and neurotransmitters, are too small to be synthesized directly by ribosomes. Instead, the ribosomes on the ER synthesize a large precursor protein that is later cut up into small peptide fragments as it traverses the Golgi.

Example: proopiomelanocortin (POMC) — a polypeptide of 241 amino acids from which is cut

As they do so, their COPII coat is removed and they may fuse together forming larger vesicles.

These fuse with the cis Golgi

Sugars are added to proteins in small packets so many glycoproteins have to undergo a large number of sequential steps of glycosylation, each requiring its own enzymes.

These steps take place as shuttle vesicles carry the proteins from cis to medial to the trans Golgi compartments.

At the outer face of the trans Golgi, vesicles pinch off and carry their completed products to their various destinations.

Mechanism 2.

In addition to the pinching off and fusing of shuttle vesicles, the cisternae of the Golgi actually migrate themselves; that is, the cis Golgi gradually migrates up the stack becoming a medial and finally a trans Golgi (depicted in the figure with red arrows).

The Inbound Path

The movement of cisternal contents through the stack means that essential processing enzymes are also moving away from their proper site of action.

Using a variety of signals, the Golgi separates the products from the processing enzymes that made them and returns the enzymes back to the endoplasmic reticulum.

This transport is also done by pinching off vesicles, but the inbound vesicles are coated with COPI (coat protein I)